PROCESS FOR THE PRODUCTION OF HIGH PURITY MELAMINE

Abstract: Improved process for the production of high purity melamine, where the melamine is obtained by conversion of urea and where the purification of melamine is performed in two distinct crystallization stages. The first crystallization consists of the production of raw melamine crystals that are washed with mother liquor coming from the second crystallization. This mother liquor is previously purified by means of a specific treatment. The second crystallization step involves the dissolution of the raw crystals with the recycled mother liquor coming from the final centrifugation, the filtration of the obtained solution, the re-crystallization, the separation by centrifugation and the washing of the crystals with demineralized water.

Description: PROCESS FOR THE PRODUCTION OF HIGH PURITY MELAMINE The present invention relates to an improved process for the production of very high purity melamine by two distinct stages of product crystallization. The invention can be applied to any known process for the production of melamine from urea, where the melamine is recovered from the reaction effluent (which contains in addition to the melamine, ammonia, carbon dioxide and by-products of the reaction) by cooling and washing with ammonia solution to recover the melamine either as a solution or as a slurry containing solid melamine particles suspended in an aqueous ammonia medium. Various procedures have being proposed to obtain, from such ammonia solution, melamine with purity equivalent to commercial standard; however it is necessary to remove from the melamine the present impurities that include the following types: a) Fine catalyst particles, typically activated silica, with reference to the catalytic processes only. b) Deammoniation products of the melamine (melam, elem, melon) . c) Hydrolysis products (typically ammeline, am elide, and melamine cyanurate) . d) Organic products of various nature, present in traces, which tend to deteriorate the colour specification of the product. The catalyst particles are insoluble and can be removed by simple filtration. The organic by-products of the type b) are lightly soluble either in alkaline or in neutral solution. The organic by-products of type c) and d) are soluble in alkaline solution and very lightly soluble in neutral solution. At the present state of the art, the removal of these by-products is obtained by filtration of a melamine solution performed in controlled conditions of temperature and pH. For that above indicated, it is preferred to perform the filtration at a pH close to neutral (7.0+1.0). A typical process for the purification of the melamine, as described in the USA patents Nr. 3.496.117 and 3.598.818 provides the recovery and the purification of the melamine through the following steps: I) Recovery of the melamine from the reaction gas by an ammonia solution. The melamine is recovered as a slurry stream (part in solution and part as suspended solid particles) with typical concentration between 4 and 6%. II) The solution of melamine is sent to some hydrocyclones where it is concentred and a slurry with a concentration of particles of 30÷40% in weight is separated from the solution. The diluited solution product inside the hydrocyclons is recycled to the cooling of the reaction gas. III) Removal of most of the ammonia and carbon dioxide from the concentrated slurry by hot stripping with steam. IV) Full dissolution of the stripped concentrated slurry in the mother liquor coming from the subsequent centrifugation (phase VII) in a dissolving vessel with long residence time, to completely dissolve the melamine. The dissolution is performed at a temperature of 100÷110°C at a neutral pH conditions obtained by addition of an organic acid or carbon dioxide. V) Filtration of the solution after addition of activated carbon. VI) Crystallization VII) Centrifugation to separate the melamine crystals, to be sent to drying, from the mother liquor to be recycled for the dissolution (phase IV) . The above procedure does not permit to produce melamine of very high purity (especially with constant purity) during the continuous operation of the plant. It has been recognized that, with the increasing of the number of the above production cycles, the characteristics of the product tend to deteriorate, up to become not controllable and off-specification. In particular it has been verified that the product tends progressively to not comply with the formaldehyde test and, in same cases, with colour standard and with the heavy metal content. The formaldehyde test consist in dissolving 26 g. of melamine in 50 g. of formaldeyde at controlled temperature: if the product meets the required purity the solution results clear. It should be noted, from the above described procedure, that the dissolution (phase IV) and filtration (phase V) steps are particulary critical for the purification of the product. The dissolution of the melamine, the control of the temperature and pH, the addition of activated carbon and the segregation of all the impurities into a solid phase are performed in this equipment. An efficient filtration is complementary to an efficient dissolution and the product can become contaminated because of an operating or mechanical failure. The situation becomes more difficult due to the possible changes of the fluids fed to the dissolving vessel (the melamine slurry and the recycled mother liquor) and because of the difficulty of checking in time the presence (also in traces) of by-products that can contaminate the final product downstream the filtration with the consequent production of off- specification product. It should also be noted that some of the by- products tend to be present also after the filtration. These by-products include impurities that are in solution during the filtration or which are formed in the liquid phase by the hydrolysis of the melamine (ammeline, ammelide, etc.). To avoid these drawbacks (as indicated in the European Patent No. 0091174) it has been proposed to perform an additional treatment consisting in the filtration of a fraction of mother liquor from the centrifugation (phase VII) . It is introduced for this reason a new step: VIII) Filtration of a fraction of mother liquor in controlled conditions of temperature and pH. In this alternate procedure the primary filtration (phase V) is performed on a solution with alkaline pH, between 7.5 and 9.0, where some of the by-products, with special reference to the melamine hydrolysis product, are partly soluble. The patent above mentioned, with a second and subsequent filtration of a fluid already filtered, seems to confirm implicitly the presence of the drawbacks above indicated and the fact that these drawbacks are due to a continuous progressive build¬ up of impurities. However this second filtration does not guarantee the production of a constant high purity melamine during the continuous operation of the plant. It is implicit, with reference to the European Patent No. 0091174 , that the melamine is produced by crystallization from a solution containing a not negligible level of impurities. In fact, the impurities soluble in alkaline environment should be present in concentration from about 6 to 13 times higher than the corresponding amount of by-products formed in the process as indicated by the need of the auxiliary filtration (VIII) of the 8% to 15% of the circulated mother liquor. It should be noted that the crystallization is performed at a sensibly lower temperature (typically between 40 and 60°C) and at a lower pH than those of the primary filtration (which is performed at a temperature between 100 and 110°C) . The pH decreases because of the vaporization of some dissolved ammonia during crystallization. In addition, since there is an evaporation of part of the liquid during the crystallization, there is a sensible increase in the concentration of the by¬ products. All this favours the segregation of impurities in the crystallization that in any case will be present in the final product through the mother liquor. The present invention eliminates the above described drawbacks and permits the production of melamine by crystallization from mother liquor with extremely low impurities content by way of a procedure that assures the absence of phenomena of progressive accumulation of by-products. According to the invention, the purification of the products is obtained through two distinct crystallization stages. The attacched block schemes show the primary crystallization (figure 1) and the final crystallization (figure 2) . With reference to figure 1, downstream the reaction (A) between urea and ammonia, the following phases are in succession: (B) Condensation of the melamine from the reaction gas with an amount of mother liquor sufficient to assure the full solubilization of the melamine. (C) Removal of insoluble impurities by filtration (primary filtration) . (D) Primary crystallization. (E) Primary centrifugation with washing of the raw crystals with purified mother liquor coming from the final crystallization. The washing of the raw melamine crystals with the mother liquor specifically treated by a purification process represent an operation of main importance for the efficacy of the procedure. The raw melamine produced in this way has a good purity, but it does not meet the formaldehyde test and the colour specification. To improve the purity of the raw melamine, the latter is subject to an additional purification stage shown in the block scheme of figure 2, where the following steps are provided. (F) Dissolution of the raw melamine in the recycled mother liquor coming from the final crystallization. (G) Second filtration (H) Final crystallization (I) Final centrifugation with washing of the product crystals with demineralized water. (K) Drying of the product crystals (J) Recycle of most of the mother liquor to the dissolving vessel with auxiliary purification of the fraction of mother liquor used for the washing of the raw melamine crystals after the primary centrifugation (E) . The advantages of the invention are the followings: a) In contrast with the known processes, the purification of the product is performed in two distinct stages and each one of them can be more easily controlled. In the first stage (primary filtration, crystallization and centrifugation) impurities insoluble in alkaline environment are removed. The reaction effluent and the treated mother liquor enter the first stage while the ammonia gas is sent to the recovery, the by-products are removed by filtration and the raw melamine is produced. The raw melamine, because of the counter current washing with a fraction of purified mother liquor coming from the 2nd step, has a high purity, but it does not meet the colour and the clarity test. For an efficient pre-purification, the washing mother liquor shall have a flow rate between 0.3 and 3 times the weight of the raw melamine and a temperature lower than 90°C, preferable between 60 and 80°C. b) The final purification stage (second filtration, final crystallization, and purification of the mother liquor used for washing the raw melamine crystals) results dedicated to the removal of the residual impurities present in traces in the raw melamine after the washing and it permits the production of the melamine with constant very high purity and with very good colour and limpidity (formaldehyde test) specification. The most critical aspect of the melamine purification is the formation, also downstream the filtrations, of by-products impurities, in addition to those present in the raw melamine. These compounds, theoretically insoluble in neutral solutions, tend to remain in solution over the solubility value according to the phenomena of supersaturation on which basis a dissolved solid can pass quickly and suddendly to the solid state. To assure the product purity, it is essential that the melamine crystallizes from a solution where these by-products are present in very small amount and in any case the accumulation of these impurities must be avoided. It should be noted that the traces of by¬ products present in the raw melamine, obtained with the procedure subject of the present invention, are essentially present in solid phase, since they derive from a crystallization process. In the re-dissolution of the raw melamine, which is performed in neutral pH environment, these impurities remain in solid state and have a size suitable for easy removal by filtration. In the known processes, used for the production of the melamine, a slurry, which contains solid melamine (typically 30÷40% wt) and any mother liquor with residual alkalinity with by-product impurities in solution and/or in supersaturation is sent to the dissolving vessel. The by-products in solution, which are fed to the dissolving vessel, tend to remain in solution because of supersaturation or to form very small size crystals that are removed by filtration with difficulty. The second cause of contamination is the hydrolysis by-product that are formed during the same process of final purification. The rates of formation of these by-products depend on the temperature and on the residence time. The formation rate increases, by increasing the temperature and the residence time. The more critical items of equipment are the dissolving vessels and the filters, which must operate at temperature sensibly high to permit the dissolution of the melamine with not excessive amounts of water, according to the solubility characteristics of the melamine. The dissolving temperature in the process generally used is about 100°C and generally less than 110°C. The dissolving temperature is associated with considerably long residence times to permit the full dissolution of the melamine and the transfer of by¬ product impurities from the liquid phase to a solid phase that can be easily removed by filtration. Two opposing conditions take place during melamine dissolution according to the present technology: while the by-products dissolved in the mother liquor pass to the solid phase, at the same time the conditions for the formation of hydrolysis by-products are favoured because of the high residence time and the high dissolving temperature. As a conseguence, the hydrolysis by-products are present in the final mother liquor in appreciable amounts, especially if the filtration is performed in alkaline environment. The content of these by-products at the start-up is limited and it permits the production of melamine which meets the specification, but continuing the operation of the plant, their concentration increases progressively and with the time deteriorates the product purity when the conditions, for a sensible formation of these by-products are realized in the crystallization. When this appears, the mother liquor becomes heavily contaminated and it may be necessary to discharge the mother liquor from the equipment with sensible economic and ecological damage. As already mentioned, in the process according to the subject invention, the dissolving vessel is fed with raw melamine crystals, with an already high degree of purity, since most of the impurities have been already removed in the primary filtration. The primary filtration, centrifugation and washing of the crystals with the purified mother liquor represent barriers that obstruct the transfer of the by-product impurities in the raw melamine crystals. It has been ascertained that the washing of the raw melamine crystals with the purified mother liquor produces an inhibiting effect on the formation and accumulation of hydrolysis by-products and of colour bodies. It has also been noted, that to obtain melamine with very high purity, the dissolution of the melamine in the purified mother liquor must be performed at temperature around 90°C and less than 100°C. At these temperatures close to 90°C, the residence time required for the dissolution of the melamine can be decreased since it is not necessary to activate the crystallization of by-products present in traces and only in solid phase. The resulting solution, intrinsically neutral, becomes fully limpid and purified after filtration. After crystallization performed at progressively decreasing temperature, high purity melamine crystals are separated by centrifugation. The crystals, after washing with demineralized water, are dried. The mother liquor is recycled to the dissolving vessel, with the exception of a fraction, from 0.3 to 3 times the weight of the raw melamine, which is purified by treatment with activated carbon largely available in the market. The treatement temperature ranges from 60 to 80°C, preferably 20°C higher than the operating temperature of the final product crystallizer. The activated carbon is sent, together with the purified mother liquor, to the washing of the raw melamine E. The activated carbon settles on the crystals and it is sent with the crystals to the melamine dissolution F where completes its purification action, and it is finally removed in the second filtration G. In the experimentation of the procedure according to the invention, changes of the high quality of the melamine product have not been observed; further, after prolonged operation, the characteristics of the melamine obtained are the following: Concentration > 99.9% in weight Turbidity absent Colour < 12 APHA The present invention has been illustrated and described according to some preferred embodiment thereof but it should be understood that construction modifications may be made by those skilled in the art without departing from the scope of the present invention.

Claims: CLAIMS : 1. Process for the production of high purity melamine derived from the reaction of urea in ammonia characterized by the fact that the product purification is performed in two distinct crystallization stages, where the washing of the crystals in the first stage is performed with a fraction of the mother liquor, coming from the final centrifugation, subject to a purification treatment. 2. Process according to the claim 1 characterized by the fact that the recovery of the melamine from the reaction effluent is performed by washing of the reaction products with an ammonia solution where the melamine is fully dissolved. 3. Process according to the claims 1 and 2 characterized by the fact that the said ammonia solution is subsequentely filtered with the removal of most of the impurities, crystallized and centrifuged to obtain raw melamine crystals. 4. Process according to the claims from 1 to 3 characterized by the fact that the washing of the raw melamine crystals is performed with a fraction of the mother liquor, coming from the final centrifugation and purified by activated carbon treatment. 5. Process according to the claims from 1 to 4 characterized by the fact that the temperature of the purification treatment with activated carbon ranges from 60 to 80°C, preferably 20°C higher than the operating temperature in the final melamine crystallization stage. 6. Process according to the claims from 1 to 5 characterized by the fact that the raw melamine crystals are dissolved into a solution of mother liquor deriving from the final centrifugation of the product at a temperature between 80 and 100°C. 7. Process according to the claims from 1 to 6 characterized by the fact that the solution obtained by dissolving the raw melamine crystals is filtered and then sent to one or more crystallizers in series, operating at temperature progressively lower that of dissolution and where the crystallization effluent is fed to a centrifuge or separator assembly to separate the mother liquor from the melamine crystals that are washed in the demineralized water. 8. Process according to the claims from 1 to 7 characterized by the fact that the mother liquor used to wash the raw melamine crystals is fed with a rate from 0.3 to 3 times the weight of the raw melamine at a temperature lower than 90°C and preferably between 60 and 80°C. 9. Process for the recovery and purification of melamine from gaseous reaction products from urea and ammonia, by crystallization from mother liquor with very low content of impurities characterized by the fact of including the following operations: a) condensation of the melamine from the reaction gas with an amount of mother liquor coming from the primary crystallization, sufficient to fully dissolve the melamine; b) removal of the insoluble impurities by filtration (primary filtration) ; c) primary cristallization; d) primary centrifugation with the washing of the raw crystals with the purified mothers liquor coming from the final crystallization; e) dissolution of the raw melamine in mother liquor recycled from final centrifugation; f) second filtration; g) final crystallization; h) final centrifugation with washing of the product crystals with demineralized water; k) drying of the product; j) recycle of most of the mother liquor to the dissolving vessel with purification treatement of the fraction of mother liquor sent to the washing of the raw melamine crystals after the primary centrifugation d) . 10. Process for the recovery and purification of the melamine according to the claim 9 characterized by the fact that the washing of the raw melamine crystals of the primary crystallization is performed with a stream of purified mother liquor at a rate from 0.3 to 3 times the weight of the melamine subject to the purification process, and at a temperature not higher than 90°C, preferably between 60°C and 80°C. 11. Process for the recovery and purification of the melamine according to the claims 9 and 10 characterized by the fact that the treatment of purification of the fraction of mother liquor used to wash the raw melamine crystals, consisting of a treatment with activated carbon at temperatures between 60 and 80°C, preferably 20°C higher than the operating temperature of the final crystallizer of the melamine where the activated carbon is added to the said mother liquor to form the stream for washing the raw melamine crystals. 12. Process for the recovery and purification of the melamine according to the claims from 9 to 11 characterized by the fact that the dissolution of the raw melamine is performed with the mother liquor obtained from the final centrifugation of the product and a temperature from 80 to 100°C.

Inventors: CASSARINO SALVATORE, GIACOBBE FRANCESCO, CASSARINO, SALVATORE, GIACOBBE, FRANCESCO
Applicants: CASSARINO SALVATORE, CONSER SPA, GIACOBBE FRANCESCO, KINETICS TECHNOLOGY, CASSARINO, SALVATORE, CONSER S.P.A., GIACOBBE, FRANCESCO, KINETICS TECHNOLOGY INTERNATIONAL S.P.A.
Application year: 1994
Main classification: C07D251
IPC codes: C07D251